The natural rubber supplied by plantations is in general too hard and not plastic enough for it to be able to be mixed and further processed directly with rubber chemicals. The same applies to types of synthetic rubber in hard formulations, e.g., copolymers of butadiene and styrene or acrylonitrile and other rubber-like co-polymers. It is, therefore, necessary for such relatively hard rubbers, which are not very plastic to be masticated, i.e., rendered plastic, before the mixing process. Mastication can take place purely mechanically without the addition of chemical auxiliary substances by working on a mill or in a kneader. However, this process, which must be carried out at a low temperature, in general takes a relatively long time. For better utilization of the processing capacities available in the plant and to save energy, masticating agents are, therefore, in general, added and the rubber is masticated at higher temperatures.
A very rapid and uniform breakdown of the rubber is achieved in this manner. The usual masticating agents are, inter alia, thiophenols, which can be substituted by various substituents on the aryl radical, and furthermore, disulfides and zinc salts of pentachlorothiophenol. Nitroso compounds and hydrazine compounds have, more-over, been used as masticating agents. However, it is also known that although the classes of compounds mentioned are already very active in small dosages, the concentrations of masticating agents should not fall below certain minimum values.
Pentachlorothiophenol (PCTP) and zinc salts thereof above all are used in practice today for mastication of high molecular weight rubbers (NR, SBR). However, the disadvantage of the use of pentachlorothiophenol is that under certain circumstances pentachlorothiophenol is capable of forming highly toxic dioxins. Other less toxic products, such as dibenzamide-diphenyl disulfide (DBD), however, have the disadvantage that they are quite expensive and, like PCTP, as a crystalline substance, they are poorly distributed homogeneously in the rubber matrix in the small amounts used which are typical of mastication. As a result, there is the risk of inhomogeneous mastication, and resulting from this, a heterogeneous molecular weight distribution of the masticated rubber.
For the preparation of rubber mixtures, i.e., mixtures of masticated rubber, which comprise all the chemicals required for processing and vulcanization, such as fillers, stabilizers, vulcanizing agents and vulcanization accelerators, as a rule three process steps are necessary: i) such as mastication of the rubber, ii) premixing of the masticated rubber for incorporation of the rubber chemicals and fillers and, as the last step, iii) addition of the vulcanizing agent into the previously mixed rubber mixture.
It is important that the three steps described are carried out separately, because other-wise the action of the masticating agent would be impaired by the rubber chemicals and the distribution of the rubber chemicals would be impeded. Furthermore, pre-mature admixing of the vulcanizing agent could trigger off an unwanted vulcanization.